In U.S. Ser. No. 753,869, filed Aug. 20, 1968 entitled "Photopolymerization of Epoxy Monomers", now U.S. Pat. No. 3,708,296 issued Jan. 2, 1973, there are disclosed novel compositions comprising various epoxy materials and certain latent curing catalysts therefor. Such compositions are photosensitive and when exposed to an energy source such as actinic radiation yield epoxy polymers which are receptive to ink and possess inherent toughness, abrasion resistance, adherence to metal surfaces, resistance to chemical attack, etc. and are thus valuable for many applications particularly those involving formation of acid and alkali resist images for chemical milling, gravure images, offset plates, stencil-making, etc.
It has now been discovered that a specific epoxide material, a copolymer of glycidyl methacrylate and allyl glycidyl ether, specially prepared, when utilized with the latent curing catalysts of said U.S. Pat. No. 3,708,296 is unique in exhibiting superior photosensitivity, adhesion and other properties, may be rapidly cured after a short exposure to an energy source without the additional application of thermal energy such as heat and is readily prepared by a process as described hereinbelow. It has further been discovered that such superior properties of said copolymeric epoxide material renders it eminently suitable for use in the recording and storage of information derived from a laser source, particularly as holograms.
U.S. Pat. No. 3,074,869 discloses epoxy resin prepolymers, including polyglycidyl methacylate, cured with certain nitrosamines via a free radical mechanism. Compositions containing this homopolymer however, following light exposure, must be heated to 140.degree. C to cure or crosslink the polymer so that an image may be developed. Further, preparation of the homopolymer is tedious requiring 3 to 4 days of heating at 50.degree. C to yield a useful polymerizable product.
The advantages of the copolymers of this invention over such prior art homopolymers in ease of preparation and rapid curing and use without subsequent heat are readily apparent and provision of such copolymers fulfills a need in the art for such products in terms of efficiency and superior properties. Moreover, as further illustrated hereinbelow, such a homopolymer does not lead to products of photosensitivity comparable to that of the copolymer of the invention.
It is known to employ various presensitized substances including silver halide film, bichromated gelatin and photopolymers such as photochromic spiropirans and compounds derived from barium acrylate, lead acrylate-acrylamide with photocatalyst solution comprising methylene blue and sodium salts of various organic acids to record laser interference patterns. Such disclosures appear in the literature at Applied Physics Letters, Vol. 15, No. 7, October, 1969 (p.201-203; Ibid., Vol. 14, No. 5, March, 1969 (p. 159-160).
None of these existing systems are without serious deficiencies either in terms of low resolution of images, instable image formation, difficulty and slowness in processing, short-life of systems employed or the use of expensive silver salts.
The present invention provides substances for this purpose that are devoid of the deficiencies listed above.